A position detector for detecting a relative positional relationship between an original form that has a position detecting mark, and a stage that supports the original form and has a reference mark. The position detector includes an illumination optical system for illuminating the position detecting mark or the reference mark using illumination light, a detection optical system for introducing reflection light that is reflected from the position detecting mark or the reference mark, to an image pickup unit, and a controller for varying at least one of a numerical aperture of the detection optical system, a numerical aperture of the illumination optical system and a wavelength of the illumination light to control contrast of a mark signal that is detected by the image pickup unit and indicates the position detecting mark or the reference mark. The relative positional relationship is detected using offsets generated in a predetermined numerical aperture of the detection optical system, a predetermined numerical aperture of the illumination optical system and a predetermined wavelength of the illumination light measured beforehand.